The design of rounds of ammunition used by artillery guns have evolved over the years to take advantage of improvements in guns and to be able to defeat the defense targets. Improvements in the designs of tank armor have called for new types of ammunition capable of penetrating and defeating such armor. For example, the HEAT (high-explosive anti-tank) shaped charge round, containing an internal explosive charge, was developed specifically as an armor piercing projectile. A more recent example is "AP shot" (armor piercing shot) in which, in one type, the ammunition round may be a high-carbon alloy steel projectile, without an explosive charge or fuse. A similar projectile is "HVAP", which is a projectile having a hard core of tungsten carbide or similar material.
A relatively recent high-velocity armor piercing ammunition is a "sabot-projectile" which consists of an outer tubular expendable discarding sabot and an inner subcaliber solid metal projectile. The sabot-projectile, when fired from the artillery gun, presents a large cross-sectional area to obtain high muzzle exit velocity for ballistic flight. The projectile, in free flight and freed from its sabot, has relatively less velocity retardation caused by air drag, etc., since its diameter is less than that of the combined sabot-projectile.
In the sabot-projectile type of ammunition round, the interlocking connection between the sabot and the projectile mu keep them together before the gun is fired, i.e., while being transported, etc., and also while in the gun bore. However, they must separate while in ballistic flight. It has been suggested that grooves, which may be axially aligned or at an angle to the axis, may be used on the external wall of the projectile or the internal wall of the sabot, or both, and elongated projections on the opposite member may fit in such grooves. Some types of sabot-projectiles have used a single direction interlocking thread and other types have used frictional engagement, i.e., the smooth outer wall of the projectile frictional fits the smooth inner wall of the sabot.
When the sabot-projectile uses a frictional fit interlock, the frictional characterization of the interlock may change with storage or in adverse environments. For example, the non-metallic parts may absorb moisture (hygroscopic distortion) or the metal parts may oxidize. The use of interlocking grooves may require special and expensive metal working. The use of a single helical thread, although relatively low in manufacturing cost, may present an unequal loading by overloading the forward end and may permit rotary slip between the sabot and projectile before they exit the gun barrel.